In general, a battery power supply unit is an electric power source supplying an energy to a related electronic device and a multicell battery pack is applied as the supply unit. By using the multicell pack, rather than a single cell, it is possible to apply a high voltage or to increase a capacity. However, since the cell itself has charge/discharge characteristics, a voltage of each cell tends to be unbalanced as time goes by.
The voltage difference between the cells in the battery pack may generate an un-balancing between the battery cells, thereby causing a capacity loss of the battery pack. With regard to this, it has been developed various battery balancing systems and methods for balancing each cell so as to prevent overcharges of all battery cells and to uniformly charge the cells.
For example, there is a method of allowing a current to flow to a high voltage cell of cells in a battery pack through a resistance and the like so as to adjust a battery balancing. Although this method is simple, it has a disadvantage that a discharge current is increased and heat is much generated if the number of unbalanced high voltage cells is increased. In addition, the method has also a disadvantage that the balance is adjusted to a cell having the lowest voltage of the cells in the battery pack.
In addition, there is a method of allowing a charge current to flow to a low voltage cell of cells in a battery pack so as to adjust a balancing. This method uses a DC-DC converter and has advantages that an efficiency is generally high and heat is less generated.
However, the method has also a disadvantage that a cell voltage of the entire battery pack becomes lower than its original lowest voltage if the number of low voltage cells is increased.
Further, there is a method of detecting a voltage irrespective of whether a balance current flows in a cell or nor when detecting voltages of cells in a battery pack. This method has disadvantages that there occurs a voltage drop due to the balance current and a voltage cannot be correctly detected in a system of adjusting a balance with a high current or a system of commonly using a path in which the balance current flows and a voltage detection path.
For example, in case that a balance current flows to a cell, a terminal voltage of the cell is complicatedly changed due to a change of the balance current. Accordingly, when there is a cell in which the balancing current flows, if a voltage is read irrespective of the balancing current, voltage values of the respective cells are differently read even though all the cells are balanced.
The above case is described with reference to FIGS. 1 and 2.
FIG. 1 is a schematic view of a system commonly using a path in which a balance current flows and a voltage detection path in a lithium ion cell battery of the prior art and FIG. 2 is a view showing voltage values read in a lithium ion cell battery of the prior art.
Referring to FIG. 1, in case that load current flows through a load device 5 in a voltage balancing adjusting apparatus according to the prior art, terminal voltages of cells are sensitively changed according to variances or magnitudes of the load current. In addition, in case that it is performed a boost balancing for a cell (B1) and a buck balancing for a cell (B2) by a balance current control section 4, when voltages of cells (B1, B2, B3, B4) are detected, voltage values of the respective cell terminals are differently read even though voltages of all cells are substantially balanced. As a result, a CPU 3 decides that each of the cells is unbalanced, and thus outputs a balance control signal to control a balance current control section 4.
Referring to FIG. 2, if the cell terminal voltages are read when the balance current does not flow in the battery balancing system, it can be seen that the terminal voltages of the cells (B1, B2, B3, B4) are constantly balanced. Accordingly, the CPU 3 decides that the voltage of the cell terminal is balanced. However, if the cell terminal voltages are read when the balance current flows, the CPU 3 decides that the voltages of the cell terminals are out of balance.
Like this, if the CPU 3 reads the terminal voltages of the cells when the balance current flows, the read voltage values of the cells may be different due to a voltage drop resulting from the balance current.
Accordingly, the battery balancing system according to the prior art has diverse condition limitations. For example, it performs a voltage balancing only when the system itself is not operated. Due to the limitations, the voltage balancing operation becomes complicated, an unnecessary time is consumed and an accuracy of the voltage balancing is decreased.
Therefore, it is required a battery balancing system or method capable of eliminating a voltage reading error of cells in a cell system, rapidly performing a voltage balancing and increasing an accuracy of a voltage balancing.